1. Field of the Invention
This invention pertains to a method and apparatus for controlling the distribution of secondary air between different fuel streams of a multiple fuel combustor.
2. Description of the Prior Art
The majority of fuel combustors in use today are designed to completely oxidize a single fuel in a complete and efficient manner. The total flow of air for oxidation to the combustor is regulated based on the oxygen content of the flue gas exiting the combustor. Air flows to a combustor in two streams, a primary air stream and a secondary air stream. The primary air stream, particularly in the application in which a granulated solid fuel is combusted in suspension within the combustor, is used to convey the fuel into the combustor. As a result, the flow characteristics of the primary air stream are dictated by the requirements of the fuel feed to the combustor. In a pulverized coal fired combustor the primary air stream will not supply adequate combustion air for complete coal burnout, thus a secondary air stream is required.
The secondary air stream is typically admitted at or nearby the fuel injecting burners in the combustor. The secondary air stream provides the balance of the combustion air necessary to adequately react the fuel in the combustor. It should be noted that in those combustors in which a primary air stream is not required, for example oil or natural gas fired applications, the balance of the combustion air is still termed the secondary air stream despite the fact that no primary air stream is present.
In recent years it has become advantageous to design and build combustors which have the capability of firing a variety of fuels. This makes possible the use of an alternative fuel should the design fuel be unavailable or unattractively priced. Typically this has been accomplished by equipping the combustor with two sets of burners, one for each type of fuel. During those periods when both sets of burners are in use, the distribution of the secondary air between each set of burners must be controlled so as to insure proper burnout of the fuels. Current practice in this regard involves manually fixing the distribution of secondary air between the burner sets based on expected firing rates and fuel types.
This method in the prior art of controlling secondary air distribution is unsuitable for use in applications in which at least one of fuels may have an unpredicably varying chemical energy content. As a result of this variation, the amount of air required to adequately react this fuel will also vary, thus resulting in an improper balance of secondary air distribution between the different fuel burner sets.
Typical applications in which a varying fuel stream may be present include the firing of wood chips or manufactured gas in conjunction with a fuel of more stable composition such as oil, natural gas or coal. Another potential application would be in the combustor section of a coal gasifier in which recycled char particles and fresh coal are fired sumultaneously to generate heat for driving the gasification process.
The recycled char in a coal gasifier is composed of unreacted carbon from the gasification reaction and inert ash particles which were originally present in the coal feed. The portion of the char composed of carbon may range from 75% to 0% with the remainder being inert ash. Should the amount of secondary air distributed to the char burners be above (or below) the optimum level, the resulting incorrect char-air mixture could cause unreacted oxygen (or carbon) to be present at the combustor section exit.
Current methods of control used on pilot scale development gasifiers involve manually setting the secondary air flow dampers to distribute the flow of secondary air between the coal and char burners based on the assumed chemical content of the char currently being fed.
In summary, the prior art methods of setting the distribution of secondary air between burner sets in a multiple fuel firing combustor are inadequate when the composition and oxygen requirement of one of the fuels varys unpredictably over time. Operation of a burner set with an excessive or inadequate amount of secondary air will result in inefficient energy utilization and/or incomplete combustion of the fuel.